The present invention relates to a method of producing a helical internal gear and, more particularly, to a method of producing a helical internal gear by roll forming, which can suitably be carried out in a cold forming operation.
It has been well known to produce a helical internal gear by cutting or by roll-forming using cold forming dies.
For instance, U.S. Pat. No. 4,452,060 discloses a prior method for forming a helical involute spline on an inner surface of a hollow blank, in which a punch provided at an outer surface with a helical involute spline is forcibly inserted and driven into a bore of the hollow blank which is supported at one end thereof by dies.
U.S. Pat. No. 4,452,060 also discloses a method for forming a helical internal gear on a blank, in which a punch provided with a helical tooth is rotated and forcibly driven into the blank using a forward extruding process.
These known methods, however, are still unsatisfactory in that they cannot completely eliminate the necessity for a cutting operation. These methods require a cutting operation for the purpose of forming a stepped entrance to facilitate an introduction of the punch into a bore of the hollow blank. In consequence, the number of steps to be employed is increased and the yield of the material is impaired, with the result that the production cost is greatly raised particularly in a mass-production situation.
Another problem is that, since such gear forming operation is carried out by the forward extruding with the punch of a leading end portion of which is unsupported and free, it is difficult to obtain products having high degree of coaxial precision.